This electrosurgical circuit monitors power consumed at the electrodes and establishes a power measurement independent of capacitance such that cables of preset impedance and the effects of high frequency current on multipliers presently used in such circuits are eliminated. It has been found that the use of certain circuit elements that are more stable give values for power consumed which when mathematically applied provide the exact power measurement of the electrosurgical generator under varying loads.
Capacitively loaded circuits, i.e. an electrosurgical generator with long leads, or laparoscopic electrosurgical instruments that capacitively couple with the trocar or other instruments, e.g. endoscopes, video etc., are difficult to accurately regulate and automatically control since the impedance signals are subject to the effects of capacitive and inductive sensitive components. In particular, multipliers and/or phase detectors introduce spurious signal abnormalities which are difficult to design out and compensate for. Because of the capacitance of long leads for high frequency power transmission, the assumption that the root mean square of voltage and current can be multiplied for accurate power determination is false. The current divider effect introduced by the leads, particularly, those which are long, results in the current at the electrosurgical generator output being unequal to the current through the load, i.e. the patient's tissue. Moreover, the variation of the tissue impedance of the patient is not approximated by the output current and to measure the voltage and current delivered at the electrodes is impractical. The multiplied root mean square values of voltage and current to obtain power are inaccurate, subject to circuit component values and tend to drift over time and are unstable at the high frequencies typically used in electrosurgery. Specifically, the phase angle between the voltage wave form and current wave form must be taken into account. Traditionally, the root mean square values of voltage and current are multiplied together and further multiplied by the cosine of the unknown phase angle. Since the phase angle is not readily available and not easily measured, no accurate way of applying the cosine of the phase angle to a control the operation of an electrosurgical generator is possible. A means to account for the phase angle and any changes thereof is not appreciated or understood. A way to circumvent the problem with a circuit that is insensitive to component capacitance is required.
U.S. Pat. No. 4,922,210 has a control for the driver circuit of a high frequency electrosurgical generator that is responsive to a resultant signal which is obtained by adding a voltage signal and an inverse current signal from the output of the electrosurgical generator. U.S. Pat. No. 4,922,210 has a positive feedback high frequency oscillator with a complementary power amplifier that derives part of input from parallel resonant circuit voltage and the remainder from series resonant circuit current. The oscillator included a voltage feedback means, current inverse feedback means, algebraic addition means and pulse converting means. No apparent recognition of the problems of inaccuracies introduced by capacitive sensitive components are noted and the measurements applied to determine the power actually delivered to the load are not correct.
As a result of manual operation problems, several attempts to provide automatic generator operation when surgical forceps contact patient tissue have been patented.
U.S. Pat. No. 3,601,126 has the amplitude of the current flowing through an electrosurgical circuit monitored and compared with a reference amplitude so that it is regulated in response to the amplitude and therefore indirectly with respect to the tissue load. In connection with desiccation in a bipolar electrosurgical system U.S. Pat. No. 4,092,986 has circuitry to reduce the output current in accordance with increasing load impedance. Therein voltage output is maintained constant while the current is decreased with increasing load impedance.
U.S. Pat. No. 4,281,373, has a DC voltage k nV.sub.S proportional to the output AC voltage which is subtracted from the sum of a fixed voltage V.sub.R and a voltage rI.sub.S proportional to the current flowing through the utilization circuit, to produce a voltage that is amplified and then used to act on the chopping supply that excites the power oscillator 5. The '373 patent does not rely on a comparison between an actual output voltage and a reference voltage to control the output voltage, but rather by adding a positive feedback it tends to raise the output voltage as the current in the utilization circuit increases.
U.S. Pat. No. 4,321,926 has a feedback system to control dosage with impedance sensing. U.S. Pat. No. 4,372,315 discloses a circuit which measures impedances after delivering a set number of radio frequency pulses on a pulse burst by pulse burst basis. U.S. Pat. No. 4,658,819 discloses a circuit wherein the power delivered to the electrode is a function of the voltage from a DC supply and the load as measured by sensors of load voltage and current. A microprocessor controller digitizes the sensing signals and computes the load impedance and actual power being delivered. The microprocessor controller accordingly repeats the measurement, calculation and correction process approximately every 20 milliseconds as long as the generator is operating.
U.S. Pat. No. 4,727,874 discloses an electrosurgical generator having high frequency pulse width modulated feedback power control. Analysis of parts of high frequency signals to determine the effects of impedance loads on the electrosurgical unit are taught. U.S. Pat. No. 4,860,745 discusses the problems encountered when turning off radio frequency power based upon measurements of the time derivative of patient tissue impedance and, instead, presents a circuit which turns off generator radio frequency power based upon fixed fractional changes in the amount of radio frequency current delivered to the patient tissue during desiccation or based upon generator sparking and harmonic frequency generation. U.S. Pat. No. 4,969,885 discloses a high frequency surgery device for cutting and/or coagulating biological tissue. In the '885 patent a blocking capacitor on the active output of the isolation transformer has its sensor positioned in the circuit after a capacitor and an output voltage comparison to a desired voltage and feedback for automatic adjustment. The '885 patent has a direct response solely to variation of a measure of the actual output voltage, for control of the energization of the power amplifier that delivers that actual output voltage. U.S. Pat. No. 4,102,341 seeks to monitor the relative difference between the current I.sub.A which flows to the knife electrode and current I.sub.P which is the current which goes to (or comes from) the patient, this difference is due to stray current. In order to do this, a voltage divider 22 is provided from which the quotient I.sub.A /I.sub.P is provided as an output which is displayed on an instrument 23. This is a problem different from those which are involved in the effects of fluctuation of the applied high frequency voltage and monitoring a display or providing alarms relating solely to variations in applied voltage.
The foregoing references include disclosures of load responsive electrosurgical generators which are regulated as a function of the tissue being treated. The references are incorporated by reference and made a part hereof.